Letter 15048

Blotched areas on Clear Anodize, Material or Plating problem [Vermont] 

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We assemble a product (peristaltic pump) that requires a half a dozen or so Clear Anodized parts. These parts are on full display on the Front Panel of our pump. Sporadically the parts exhibit mottled or blotched areas. On the thin parts it appears on both sides of the part in corresponding areas. It almost appears that the indications are in the metal. The material is Aluminum 6061-T6 and the anodize process is Sulfuric Anodize Per Mil-A-8625 [link is to info about spec at TechStreet] Type ll Class 1. There is also an etch performed before the anodize process to give it a satin appearance. The parts are then hot water sealed at the end of the process. What can I do to assure a uniformed look. I can change material/temper but I still need a satin look. Note: These parts are not tumbled or glass beaded at any time. They are handled with cotton gloves to prevent contamination.

Jeff Cogger
- Vergennes,VT, USA

Quality Control is your answer. Be sure the deoxidizer is a mixed acid system that works on 6061, be sure the dissolved aluminum in the anodizing solution is under 12 gm/L as it will occlude in the pores and give splotchy finish. Be sure there is a 5% Nitric Acid dip between the anodizing and the seal to help flush out the viscous Sulfuric Acid so that the seal can get into the pores and hydrate the pores. Be sure the seal is deI water and that no calcium tap water has dragged in, be sure the seal pH is help in spec.

If there is zinc anywhere in the line from 7000 alloys or if there is calcium from hard water, then you may expect these elements to form compounds that occlude in the pores and/or lay on the surface and cause smears and white spots.

Take two aspirin and call me in the morning.

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

There are several "satin" etches that will give you the glass bead appearance without the mechanical damage that blasting can give you. Anoplate calls theirs "606 Etch" and it's typically used prior to black anodize to give a non-reflective, optical black coating, however, works beneath straight sulfuric anodize as well.

The advice from Robert Probert is quite good for your anodizer, however, doesn't help the parts' manufacturer much! If you'd like some samples of Anoplate's "606 Etch" than Clear Anodize, E-mail me with your mailing address and I'll get you some sample coupons.

Milt Stevenson, Jr. Anoplate Corp Syracuse, NY, USA

Thank you for your response Robert. One detail I forgot to mention is that all the indications are in the same place on each part. They are not areas that would entrap the solution or hold a drop, or dwell during drying. Moreover if you were to imagine a length of bar stock and line these parts in a row as they were machined, the blotches would run in a straight line parallel to the length of the bar. It is as though there was a soft spot running through the bar that anodized differently. It is like cutting paper dolls, all the parts have the same blotches in the same spots. Some however are worst than others. This may be a coincidence, because I get similar indications on other parts using the same process but in random areas.

Jeff Cogger
- Vergennes,VT, USA

We had the same problem with a forged part on a racecar, and through an aerospace customer determined that the forging process resulted in a slightly non-homogeneous material makeup, resulting in dull areas. This accounted for the consistency of the 'flaws'. Before this, we tried everything with our anodizer, as well as changing out all of our machining fluids and cleaning chemicals, which all helped us get in better sync with our anodizer, resulting in better quality parts overall. But in the end it was the forging that was causing the problems.

Josh Poertner
- Speedway, IN, USA

Oh man.. please don't get me started again on the inconsistancies of aluminum produced by the big manufacturers! See letter # 14229. I could gripe about this for hours.....

Marc Green anodizer - Boise, ID, USA

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